Composite electrode materials for fluoride-ion electrochemical cells

ABSTRACT

The present disclosure relates to a method of making core-shell and yolk-shell nanoparticles, and to electrodes comprising the same. The core-shell and yolk-shell nanoparticles and electrodes comprising them are suitable for use in electrochemical cells, such as fluoride shuttle batteries. The shell may protect the metal core from oxidation, including in an electrochemical cell. In some embodiments, an electrochemically active structure includes a dimensionally changeable active material forming a particle that expands or contracts upon reaction with or release of fluoride ions. One or more particles are at least partially surrounded with a fluoride-conducting encapsulant and optionally one or more voids are formed between the active material and the encapsulant using sacrificial layers or selective etching. When the electrochemically active structures are used in secondary batteries, the presence of voids can accommodate dimensional changes of the active material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Patent Application No.62/434,611, entitled “COMPOSITE ELECTRODE MATERIALS FOR FLUORIDE-IONELECTROCHEMICAL CELLS,” filed Dec. 15, 2016, and U.S. Patent ApplicationNo. 62/453,295, entitled “CORE SHELL,” filed Feb. 1, 2017, each of whichis incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant Number80NMO0018D0004, awarded by NASA (JPL). The government has certain rightsin the invention.

TECHNICAL FIELD

The present disclosure relates to electrochemically active materials,and more particularly to fluoride ion battery systems that includeelectrode materials with tailored structures and compositions to improvebatter performance. More specifically, this disclosure relates tocore-shell nanoparticles, methods for making the same, and use of thesame in electrochemical cells.

BACKGROUND

Metal nanoparticles are highly desirable for use in a number ofapplications including as catalysts, and as electrode materials forbatteries. However, the use of metal nanoparticles may be limited by thesystem operating conditions or other factors. For example, fluorideshuttle batteries are of growing interest as an alternative tolithium-ion batteries. However, the materials available for use influoride shuttle battery systems are limited, due in part to operatingconditions that are detrimental to many materials that could otherwisebe included in the fluoride shuttle battery electrodes.

Fluoride-ion batteries are electrochemical cells that operate viafluoride-mediated electrode reactions (i.e. accommodation or release offluoride ions at the electrode upon charge or discharge, often through aconversion-type reaction). Such electrochemical cells can offer greaterenergy density, lower cost and/or improved safety characteristicscompared to lithium and lithium-ion batteries. Fluoride-ion systems havebeen demonstrated in the solid state, for example, in U.S. Pat. No.7,722,993 to Potanin, which describes an embodiment of a secondaryelectrochemical cell where fluoride ions are reversibly exchangedbetween anode and cathode during charge-discharge cycles, with theseelectrodes in contact with a solid-state fluoride-conductingelectrolyte. Potanin describes solid state electrolytes containingfluorides of La, Ce or the compound fluorides based on them, togetherwith an alloying additives, such as fluoride/fluorides of alkaline-earthmetals (CaF2, SrF2, BaF2) and/or fluorides of alkaline metals (LiF, KF,NaF) and/or alkaline metal chlorides (LiCl, KCl, NaCl), as well as awide range of other compound fluorides. However, such electrochemicalcells operate usefully only above room temperature (e.g. 150° C.) due tothe limited conductivity of the solid-state electrolyte.

Attempts have also been made to provide fluoride ion-basedelectrochemical systems capable of using liquid electrolytes. Forexample, US 2011-0143219 A1 by Weiss et al. and U.S. Pat. No. 9,166,249by Darolles et al. disclose fluoride-ion battery configurations selectedto include a solvent-borne fluoride salt that is at least partiallypresent in a dissolved state in the electrolyte. However, for manyapplications the chemical reactivity of the electrode materials with theliquid electrolyte is significant, and these liquid electrolyte systemsdo not provide sufficiently reliable high discharge and/or high capacityoperation.

SUMMARY

The following presents a simplified summary of one or more aspects ofthe present disclosure in order to provide a basic understanding of suchaspects. This summary is not an extensive overview of all contemplatedaspects and is intended to neither identify key or critical elements ofall aspects nor delineate the scope of any or all aspects. Its purposeis to present some concepts of one or more aspects in a simplified formas a prelude to the more detailed description that is presented later.

In some embodiments, the present disclosure is directed to anelectrochemically active structure comprising: a core comprising anactive material; and a fluoride-containing shell at least partiallysurrounding the active material.

In other embodiments, the present disclosure is directed to a method ofmaking coated metal nanoparticles, the method comprising: a) providing awater/metal nanoparticle mixture; b) exposing the water/metalnanoparticle mixture to an inert atmosphere; and c) forming afluoride-containing shell around a metal nanoparticle core.

These and other aspects of the invention will become more fullyunderstood upon a review of the detailed description, which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a cross section of a core-shell nanoparticle including acore comprising a metal nanoparticle, and a shell comprising a metalhalide or a metal oxyhalide in an aspect of the present disclosure.

FIG. 1B outlines a general route to “yolk-shell” composites using asacrificial inorganic “middle” layer.

FIG. 1C describes a route to “yolk-shell” composites using a sacrificialpolymer “middle” layer.

FIG. 1D delineates an alternative route a route to “yolk-shell”composites in the absence of a sacrificial “middle” layer.

FIG. 1E depicts a variation on “sacrificial” syntheses of “yolk-shell”composites, whereby nanoparticles of the active material are grown onthe surface of a “sacrificial” material

FIG. 1F outlines the growth of active material in the internal structureor pores of a pre-formed “sacrificial” material

FIG. 1G outlines the growth of active material in the internal structureor pores of a pre-formed “sacrificial” material where the shellconstituents are chosen to be electrochemically-inactive at theelectrochemical reaction potentials of interest.

FIG. 2 is a schematic illustration of a fluoride ion electrochemicalcell in an aspect of the present disclosure.

FIG. 3 shows an XRD spectrum of isolated copper nanoparticles ofComparative Example 1, with no shell, immediately after synthesis andisolation (“as-made”).

FIG. 4 shows stacked XRD spectra of isolated copper nanoparticles ofComparative Example 1, with no shell, as-made and after exposure to airfor 4 days and 9 days.

FIG. 5 shows an XRD spectrum of Cu—LaF₃ core-shell nanoparticles ofExperimental Example 1 as synthesized in an aspect of the presentdisclosure.

FIG. 6 shows stacked XRD spectra of Cu—LaF₃ core-shell nanoparticles ofExperimental Example 1 after exposure to air for 9, 16, and 23 days.

FIGS. 7A and 7B are transmission electron microscopy (TEM) images ofCu—LaF₃ core-shell nanoparticles of Experimental Example 1, as-made.

FIG. 8A shows a high-resolution TEM image of Cu—LaF₃ core-shellnanoparticles of Experimental Example 1, indicating the Cu (core) andLaF₃ (shell) areas. FIGS. 8B and 8C show zoomed-out images of the samenanoparticles.

FIG. 9 is a schematic of illustration of a fluoride ion electrochemicalcell including the Cu—LaF₃ core-shell nanoparticle of ExperimentalExample 1 as an active material in the negative electrode (anode) in anaspect of the present disclosure.

FIG. 10A is a plot of voltage as a function of specific capacity forelectrochemical tests of a half cell battery including the Cu—LaF₃core-shell nanoparticle of Experimental Example 1 as an active materialin an electrode in an aspect of the present disclosure.

FIG. 10B is an X-ray diffraction spectrum of the electrode of the halfcell battery test of FIG. 10A, measured under initial conditions, afterdischarge, and after charge.

FIG. 11 shows an XRD spectrum of nanoparticles of Comparative Example 2as synthesized.

FIG. 12 shows stacked XRD spectra of nanoparticles of ComparativeExample 2 after exposure to air for 8, 15, and 22 days.

FIG. 13 is a TEM image of the nanoparticles of Comparative Example 2showing inhomogeneous, partial coverage of copper nanoparticles withLaF₃, as well as LaF₃ that is not associated with copper nanoparticles.

FIG. 14 shows LaF₃/Cu and Cu thin-film configurations and cyclicvoltammetry data.

FIG. 15A shows a cyclic voltammogram for a LaF₃/Cu double-layered thinfilm. FIGS. 15B and 15C show x-ray photoelectron spectroscopy (XPS) datafor C, La, 0, F, and Cu at various length of etching times at thevoltages 1 and 2 indicated in FIG. 15A.

DETAILED DESCRIPTION

In general, the present disclosure is related to electrochemicallyactive materials, and fluoride-ion battery systems that includeelectrode materials with tailored structures and compositions to improvebattery performance. In some aspects, the present disclosure is relatedto core-shell nanoparticles, devices incorporating the core-shellnanoparticles, as well as methods of making and using the core-shellnanoparticles and devices including the core-shell nanoparticles.

Primary and secondary electrochemical cells, such as batteries,utilizing fluoride ion charge carriers, active electrode materials, andsuitable liquid electrolytes can provide an alternative to conventionalstate of the art lithium batteries and lithium ion batteries. Suchfluoride-ion battery (FIB) systems can operate usefully at roomtemperature while utilizing fluoride anions carried in a liquidelectrolyte as at least some of the charge carriers in anelectrochemical cell. The FIB system has an anode and cathode physicallyseparated from each other, but in common contact with a fluoride ionconducting electrolyte. The anode is typically a low potential elementor compound, and can be a metal, metal fluoride, or intercalatingcomposition. Similarly, the cathode can be element or composition, andcan be a metal, metal fluoride, or intercalating composition that has ahigher potential than the anode. Fluoride ions (F⁻) in the fluorideconducting electrolyte go from the cathode to the anode during dischargeand from the anode to the cathode during the charge of the battery:

Discharge:Anode: MF_(x) +nF−→MF_(x+n) +ne−  (Fluoride ion accommodation,oxidation)Cathode: MF_(y) +ne−→MF_(y−n) +nF−  (Fluoride ion release, reduction)

During charge, the reverse reactions occur.

For example, a FIB cell reaction based on fluoride anion transferbetween Ca and Cu, both metals capable of forming metal fluorides, mightbe:

Discharge:Ca+CuF₂→CaF₂+CuCharge:CaF₂+Cu→Ca+CuF₂

Two major challenges exist to enable stable, reliable long-term cyclingof FIB electrodes. Firstly, reversibility of the electrochemicalreactions above is observed when the metal or metal fluoride activematerials are nano-sized (i.e. at least one of the particle sizedimensions is less than 1 μm). However, particles with such smalldimensions have high surface energies and are often reactive with theelectrolyte components (e.g. F⁻) to give undesirable side-reactionsincluding “self-discharge” (i.e. a chemical reaction such asM+nF⁻→MF_(n) that does not general electrical current). What is neededis formation of a coating, shell, layer or the like to encapsulate theactive material particles while still permitting passage of F⁻ ions whendesired (i.e. during electrochemical charge or discharge) Theencapsulating material can also protect the active materials from suchside-reactions, enabling long-term cycling stability of these electrodematerials.

Secondly, such electrochemical reactions are conversion processes, withthe structure of the metal or metal fluoride being broken down duringthe electrochemical process and reformed as the metal fluoride or metal,respectively, during the process. This conversion process results in asignificant volume change between charged and discharged states of theactive material, as indicated by the examples given in Table 1 below:

TABLE 1 Volume change for metal to metal fluoride conversion VolumeChange During Conversion Metal Metal Fluoride M + nF⁻ → MF_(n) Fe FeF₃311% Pb PbF₂  73% Bi BiF₃ 134% Co CoF₃ 351% Cu CuF₂ 238% Sn SnF₂ 113% LaLaF₃  46% Ca CaF₂  −5% Mg MgF₂  42% Li LiF −24%

Such significant volume changes limit usefulness of conformal protectivecoatings encapsulating an FIB electrode material particle, since oneparticular state of charge will not necessarily be conformal with theparticle in a different state of charge, due to the volume changes. Whatis needed are compositions and processes that protect the electrodeactive material from side reactions with the electrolyte, allow ionconduction through an encapsulant, and have sufficient void space withinthe encapsulant and/or encapsulant expansion/contraction properties toaccommodate the volume changes of the active material during charge anddischarge without permitting direct contact between the active materialand the electrolyte. In some embodiments, sufficient void space may beno void space. Such compositions and their preparation are outlinedbelow.

In some embodiments as shown in FIG. 1A, the core-shell nanoparticlesinclude a core that comprises a metal or metal alloy (“Me”), and a shellthat comprises a metal halide or a metal oxyhalide. The metal of thecore may be the same as the metal of the metal halide shell. In someembodiments, the metal of the core and the metal of the metal halide ormetal oxyhalide shell are different metals. The core-shell nanoparticlesof the present disclosure may be incorporated into a variety of methodsand applications including, but not limited to, electrodes for use inelectrochemical cells, including fluoride shuttle batteries as shown inFIG. 2.

The metals or metal alloys used to form the core include, but are notlimited to, iron nanoparticles, cobalt nanoparticles, nickelnanoparticles, copper nanoparticles, lead nanoparticles, and alkalineearth metal nanoparticles. In a preferred embodiment, the metalnanoparticles are selected from the group consisting of cobaltnanoparticles and copper nanoparticles. The metals used to form the coremay be synthesized by mixing a metal precursor solution with a reducingagent to form metal nanoparticles.

In some embodiments, the metal nanoparticles used to form the core maybe synthesized in the presence of a stabilizer that prevents orotherwise inhibits oxidation of the metal nanoparticles duringsynthesis, and is readily removable from the metal nanoparticles priorto formation of the metal halide or metal oxyhalide shell thereon. Forexample, bulky polymers such as polyvinylpyrrolidone (molecular weightof 55,000 g/mol) used during metal nanoparticle synthesis inhibitoxidation of metal nanoparticles. However, such stabilizers are notreadily removable from the metal nanoparticles following synthesis.Without being limited to any particular theory, residual stabilizer canform an additional layer between the core formed by the metalnanoparticles and the metal halide or oxyhalide shell that detracts fromthe performance of the core-shell nanoparticle in the desired system.For example, it is desirable to maintain the conductivity of core-shellnanoparticles used as electrode material in an F-Shuttle battery.However, core-shell materials including an additional layer of residualstabilizer between the core and the shell will likely result in anincreased space between electrode materials; the additional layer ofresidual stabilizer and/or the resulting increased space may decreasethe conductivity of the core-shell material. Without wishing to be boundto any particular theory, the additional layer of stabilizer may impedecontact between the core and the shell to conduct fluoride ion, whilethe absence of the stabilizer may increase the likelihood of conductingfluoride ion from the core to the shell.

Therefore, a stabilizer may be used in the synthesis of the metalnanoparticles used to form the core that is readily removable therefromto minimize the amount of stabilizer on the surface of the core prior toformation of the metal halide or metal oxyhalide shell directly thereon.In a non-limiting example, the one or more stabilizers that may be usedin the synthesis of the metal nanoparticles includes a molecular weight(either individually or a weight average) of less than 1000 g/mol,optionally less than 500 g/mol, optionally less than 375 g/mol, andoptionally less than 350 g/mol. Illustrative examples includehexadecyltrimethylammonium bromide (CTAB) with a molecular weight of 364g/mol, citric acid with a molecular weight of 192 g/mol, and mixturesthereof.

In some embodiments, the shell of the core-shell nanoparticles may beformed by mixing isolated metal nanoparticles used to form the corewith, for example, a metal salt solution and a halide salt solution thatreact to form the metal halide shell on the core. The shell is depositeddirectly on the metal core and may entirely surround the core as shownin FIG. 1A. In some embodiments, the metal salt used to form the shellis selected from the group consisting of alkali metal salts, alkalineearth metal salts, and transition metal salts. In certain embodiments,the metal salt used to form the shell is a transition metal salt. Incertain embodiments, the metal salt used to form the shell is selectedfrom the group consisting of lanthanum salts, cerium salts, andmagnesium salts. In certain embodiments, the metal salt used to form theshell is selected from the group consisting of lanthanum salts andcerium salts. In certain embodiments, the metal salt is a lanthanumsalt. In a preferred embodiment, the lanthanum salt is lanthanumnitrate. In some embodiments, the halide salt is sodium fluoride. In anon-limiting example, the shell comprises a metal fluoride or metaloxyfluoride containing material (i.e. CeF₃, CeOF, LaOF, LaF₃).

In other embodiments, the core (or electrode active material) may beseparated from the shell (or encapsulant) by a void space. Compositionsand processes according to such embodiments may protect the electrodeactive material from side reactions with the electrolyte, allow ionconduction through an encapsulant, and have sufficient void space withinthe encapsulant and/or encapsulant expansion/contraction properties toaccommodate the volume changes of the active material during charge anddischarge, without permitting direct contact between the active materialand the electrolyte.

The terms core and electrode active material are used interchangeablyherein. Similarly, the terms shell and encapsulant are usedinterchangeably herein.

In other embodiments, the present disclosure is directed to an electrodecomprising the core-shell nanoparticles disclosed herein. All aspectsand embodiments described with respect to the core-shell nanoparticlesand methods of making thereof apply with equal force to the electrode.In a non-limiting example, the electrode is part of an F-shuttle batterysystem.

An “inert atmosphere” refers to a gaseous mixture that contains littleor no oxygen and comprises inert or non-reactive gases or gases thathave a high threshold before they react. An inert atmosphere may be, butis not limited to, molecular nitrogen or an inert gas, such as argon, ormixtures thereof.

A “reducing agent” is a substance that causes the reduction of anothersubstance, while it itself is oxidized. Reduction refers to a gain ofelectron(s) by a chemical species, and oxidation refers to a loss ofelectron(s) by a chemical species.

A “metal salt” is an ionic complex wherein the cation(s) is(are) apositively charged metal ion(s) and the anion(s) is(are) a negativelycharged ion(s). “Cation” refers to a positively charged ion, and “anion”refers to a negatively charged ion. In a “metal salt” according to thepresent disclosure, the anion may be any negatively charged chemicalspecies. Metals in metal salts according to the present disclosure mayinclude but are not limited to alkali metal salts, alkaline earth metalsalts, transition metal salts, aluminum salts, or post-transition metalsalts, and hydrates thereof.

“Alkali metal salts” are metal salts in which the metal ions are alkalimetal ions, or metals in Group I of the periodic table of the elements,such as lithium, sodium, potassium, rubidium, cesium, or francium.

“Alkaline earth metal salts” are metal salts in which the metal ions arealkaline earth metal ions, or metals in Group II of the periodic tableof the elements, such as beryllium, magnesium, calcium, strontium,barium, or radium.

“Transition metal salts” are metal salts in which the metal ions aretransition metal ions, or metals in the d-block of the periodic table ofthe elements, including the lanthanide and actinide series. Transitionmetal salts include, but are not limited to, salts of scandium,titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper,zinc, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium,rhodium, palladium, silver, cadmium, lanthanum, cerium, praseodymium,neodymium, promethium, samarium, europium, gadolinium, terbium,dysprosium, holmium, erbium, thulium, ytterbium, lutetium, hafnium,tantalum, tungsten, rhenium, osmium, iridium, platinum, gold, mercury,actinium, thorium, protactinium, uranium, neptunium, plutonium,americium, curium, berkelium, californium, einsteinium, fermium,mendelevium, nobelium, and lawrencium.

“Post-transition metal salts” are metal salts in which the metal ionsare post-transition metal ions, such as gallium, indium, tin, thallium,lead, bismuth, or polonium.

A “halide salt” is an ionic complex in which the anion(s) is(are) halideion(s), including but not limited to fluoride ion(s), chloride ion(s),bromide ion(s), and iodide ion(s). A “fluoride salt” is an ionic complexin which the anion(s) is(are) fluoride ion(s). According to the presentdisclosure, the cation of the halide salt or the fluoride salt may beany positively charged chemical species.

A “metal fluoride” is an ionic complex in which the cation is a metalion and the anion(s) is(are) fluoride ion(s). According to some aspectsof the present disclosure, the metal salt and the fluoride salt react tocreate a metal fluoride shell around the metal nanoparticle core.Similarly, a “metal halide” is an ionic complex in which the cation is ametal ion and the anion(s) is(are) halide ion(s).

A “fluoride-containing” salt is an ionic complex in which the anion(s)contain fluoride ion but are not limited to being solely fluoride.Instead, “fluoride-containing” salts include ionic complexes where theanion(s) contain fluoride itself in complex with other ions or atoms.“Fluoride-containing” salts suitable for use in aspects of the presentdisclosure include those known to persons of ordinary skill in the art,including, but not limited to, fluoride salts, non-metal fluoroanionssuch as tetrafluoroborate salts and hexafluorophosphate salts, andoxyfluoride salts. In some aspects of the present disclosure, thefluoride-containing salts may include quaternary ammonium fluorides andfluorinated organic compounds. According to some aspects of the presentdisclosure, the metal salt and the fluoride-containing salt react tocreate a fluoride-containing shell around the metal nanoparticle core.

The term “electrode” refers to an electrical conductor where ions andelectrons are exchanged with an electrolyte and an outer circuit.“Positive electrode” and “cathode” are used synonymously in the presentdescription and refer to the electrode having the higher electrodepotential in an electrochemical cell (i.e. higher than the negativeelectrode). “Negative electrode” and “anode” are used synonymously inthe present description and refer to the electrode having the lowerelectrode potential in an electrochemical cell (i.e. lower than thepositive electrode). Cathodic reduction refers to a gain of electron(s)of a chemical species, and anodic oxidation refers to the loss ofelectron(s) of a chemical species. Positive and negative electrodes ofthe present invention may be provided in a range of usefulconfigurations and form factors as known in the art of electrochemistryand battery science, including thin electrode designs, such as thin filmelectrode configurations. Electrodes are manufactured as known in theart, including as disclosed in, for example, U.S. Pat. No. 4,052,539,and Oxtoby et al., Principles of Modern Chemistry (1999), pp. 401-443.

The term “electrochemical cell” refers to devices and/or devicecomponents that convert chemical energy into electrical energy or viceversa. Electrochemical cells have two or more electrodes (e.g., positiveand negative electrodes) and an electrolyte, wherein electrode reactionsoccurring at the electrode surfaces result in charge transfer processes.Electrochemical cells include, but are not limited to, primarybatteries, secondary batteries, and electrolysis systems. General celland/or battery construction is known in the art (see, e.g., Oxtoby etal., Principles of Modern Chemistry (1999), pp. 401-443).

“Electrolyte” refers to an ionic conductor which can be in the solidstate, the liquid state (most common), or more rarely a gas (e.g.,plasma).

I. Core-Shell Nanoparticle Comprising a Metal Core and a Metal Halide orMetal Oxyhalide Shell

In an embodiment, a core-shell nanoparticle is provided that comprises ametal core that is surrounded by a metal halide or a metal oxyhalideshell.

In an illustrative example, the core-shell nanoparticle may be includedin an electrode of a rechargeable battery, such as an F-shuttle battery.For example, it is difficult to use metal nanoparticles in the electrodeof an F-shuttle battery as the metal is exposed to conditions that leadto undesired oxidation or dissolution of the metal. Accordingly, ahalide shell is provided that is tailored to protect the metal corenanoparticle from the environment of the electrode while maintaining thedesired performance of the metal nanoparticle. In a non-limitingexample, the core may comprise copper metal and the shell may compriseLaF₃.

A method of making the core-shell nanoparticle may comprise providing afirst mixture comprising a metal nanoparticle and a reducing agent, andmixing the first mixture with a solution comprising a metal salt and ahalide salt to form a metal halide or oxyhalide shell on the metalnanoparticles.

I(a). Synthesis and Isolation of the Metal Core

In general, metal nanoparticles for use as the metal core may besynthesized by reacting a metal salt solution with a reducing agent inthe presence of one or more stabilizers. In an illustrative example, themetal salt solution comprises a copper (II) nitrate hemipentahydrate(Cu(NO₃)₂.2.5H₂O) as the metal salt. The metal salt is mixed with CTABand water, and the pH of the mixture may be adjusted to a pH of about10-11, with for example, ammonium or sodium hydroxide.

Prior to addition of the reducing agent to the metal salt solution, thereducing agent may be mixed with one or more stabilizers and water, andmixed for a period of time, such as twenty minutes, prior to combiningwith the metal salt solution. The reducing agent is selected from thegroup consisting of hydrazine, sodium borohydride, sodiumcyanoborohydride, sodium dithionate, sodium dithionite, iron (II)sulfate, tin (II) chloride, potassium iodide, oxalic acid, formic acid,ascorbic acid, thiosulfate salts, dithionate salts, phosphorous acid,phosphite salts, and hypophosphite salts. In a preferred embodiment, thereducing agent is hydrazine.

The metal salt solution and the reducing agent are combined to form themetal nanoparticles. Synthesis of the metal nanoparticles is performedin an atmosphere free of oxygen. Illustrative examples of atmospheresfree of oxygen include, but are not limited to, nitrogen, argon, helium,hydrogen, and mixtures thereof. Following synthesis, the metalnanoparticles are isolated from the synthesis solution. It is to beunderstood that the method of isolating the metal nanoparticles is notlimited, and may include one or more techniques such as filtering,decanting, and centrifuging. The metal nanoparticles may be washed oneor more times with a solvent, such as ethanol, to remove any residualstabilizer or other organic material from the surface thereof.

I(b) Shell Formation

In general, the isolated metal nanoparticles may be redispersed in anaqueous solution containing additional reducing agent under anatmosphere that is free of oxygen. The mixture containing the isolatedmetal nanoparticles and reducing agent is then mixed under an atmospherefree of oxygen with a metal salt solution and a halide salt solutionused to form the metal halide shell on the metal nanoparticle core. Themetal salt solution and the fluoride salt solution used to form theshell may be added sequentially to the nanoparticle mixture, or themetal salt solution and the fluoride salt solution used to form theshell may be added simultaneously to the nanoparticle mixture.

II “Yolk-Shell” Nanoparticles (i) Protective, Fluoride Ion-ConductingCoatings

Useful protective encapsulating coatings include fluoride-ion conductingphases that are chemically and electrochemically stable in the presenceof a liquid FIB electrolyte. Such phases permit the exchange of F−between the electrolyte and the active material. Suitable phases areknown in the art and are described, for example, in “The CRC Handbook ofSolid State Electrochemistry”, Chapter 6 (CRC, 1997, P. J. Gellings andH. J. M. Bouwmeester, Eds.), Sorokin and Sobolev, CrystallographyReports 2007, 52, 5, 842-863, Sobolev et. al., Crystallography Reports2005, 50, 3, 478-485, and Trnovcova et. al., Russian Journal ofElectrochemistry, 2009, 45, 6, 630-639. These include, for example,crystalline phases such as LaF₃, CaF2, SnF₂, PbF₂, PbSnF₄, analogousdoped and/or solid solution phases (e.g. La_(0.9)Ba_(0.1)F_(2.9),Ca_(0.8)Y_(0.2)F_(2.2), Ca_(0.5)Ba_(0.5)F₂, andPb_(0.75)Bi_(0.25)F_(2.25)), glassy phases such as 35InF₃.30SnF₂.35PbF₂,and mixed fluoride/other anion phases such as LaOF. For the purposes ofthis disclosure, any material or phase that permits the exchange of F−between the electrolyte and the active material, with bulk ionicconductivity above 10-10 S/cm at 298K is within the scope of theinvention. These phases are chosen with constituents selected to beelectrochemically stable at the potentials required for reaction of thespecies contained within the coatings by considering the standard redoxpotentials of the shell constituents and the inner species available instandard texts. See the example of FIG. 1G for a more detaileddiscussion of the selection of coating constituents in this regard.

Alternative protective coatings include polymers that are conducting forfluoride ions, for example boronate-functionalized polymers,alkylammonium-functionalized polymers, or those featuring suitablefunctional groups such as those described in Gorski et. al., Anal. Chim.Acta 2009, 633, 181-187 and Gorski, et al., Anal. Chim. Acta 2010, 665,39-46.

The thickness of the protective coating is chosen so that exchange of F−between the electrolyte and the active material occurs on a timescalesuch that charge/discharge of the electrochemical cell can be achievedat suitable rates of operation around 298K (e.g C-rate, corresponding tofull charge or discharge of the energy stored in the electrochemicalcell in one hour), and will depend on the ionic conductivity of thecoating material or phase. For example, a coating of LaF₃ is mostusefully between 1-200 nm thick. More generally, the coating thicknesscan be from about 1 nm to about 1 μm.

The coating can be made by any suitable method of synthesis. These mayinclude solution chemistry techniques such as the formation of thecoating by precipitation of a solid from a solution containing thefluoride or its constituent precursors, sol-gel or other soft chemistryor “chimie douce” methods, hydrothermal synthesis, vacuum methods suchas chemical vapor deposition, physical vapor deposition, sputtering,laser ablation and molecular beam epitaxy, electrochemical deposition,or fluorination of a material after deposition by reaction with afluorine source. For example, one preferred method for the preparationof a LaF₃ coating is a sol-gel synthesis similar to those described inRudiger and Kemnitz, Dalton Trans., 2008, 1117-1127 and Fujihara et al.,J. Ceram. Soc. Japan, 1998, 106, 124-126, using soluble lanthanum andfluorine sources in a suitable solvent (for example, La(CH₃COO)₃ andCF₃COOH in water). The coating as prepared may, optionally, be subjectedto elevated temperature either in air or inert gas such as Ar for anannealing step, as desired. For example, a LaF₃ coating prepared by thesol-gel method may be heated to 500° C. in air to anneal the coating andassist with removal of impurities such as solvent. In this manner,fluoride-conducting coating phases may be synthesized as desired byadjusting the precursor materials, their stoichiometric ratios, and thepost-initial reaction annealing step.

In another example, the LaF₃ coating can be obtained from precipitation,by slowly adding NH₄F into La(NO₃)₃ aqueous solution with nanoparticlesof the core material suspended therein. Since LaF₃ is extremelyinsoluble in water, its crystallization will start on the surface of thesuspended nanoparticles.

Alternatively, a sol-gel approach can be used to make a La₂O₃ coating,followed by post-fluorination using F₂ or HF to convert a substantialportion of this oxide to LaOF and/or LaF₃.

Fluoride-conducting encapsulants and/or coating phases and materials maybe prepared on a three-dimensional structure (e.g. a metal or metalfluoride nanoparticle, or aggregate of nanoparticles), a two-dimensionalstructure (e.g. a metal or metal fluoride thin film), or aone-dimensional structure (e.g. a fiber or tube of metal or metalfluoride) as required. Similarly, fluoride-conducting phases may beprepared on the external and/or internal surfaces of complex micro- ormesoporous structures such as a zeolite or highly ordered templatedmaterial. This can include, but is not limited to mesoporous silicassuch as MCM-41 or SBA-15, or metal-organic frameworks or similarcoordination polymers.

(ii) Active Materials Encapsulated within a Fluoride-Ion ConductiveCoating

Useful structures and compositions include those in which a metal ormetal fluoride is encapsulated within a fluoride-ion conductive coating(as described in (i) above) such that there exists sufficient void spacewithin the encapsulation for the volume change between metal and metalfluoride phases (or, between a lower-valent metal fluoride species Mfmand a higher-valent metal fluoride species MFn where n>m for the samemetal M) to be accommodated upon conversion without rupture of thecoating phase or material. Such structures and compositions are sized tofit within an fluoride-conducting encapsulant, in certain cases with atleast enough void space available for up to 100% of the encapsulatedmetal atoms to be converted to the appropriate metal fluoride phase(e.g. for the process Fe→FeF₃ at least 211% void space is requiredcompared to the starting volume of Fe, from Table 1). In other cases,the degree of conversion may be controlled electrochemically (e.g. bycontrolling the voltage limits and/or charge/discharge capacity) so thatthe encapsulant does not rupture during cycling in the cases wherebythere is not enough void space for 100% conversion to be achieved. Inaddition, structures and compositions are also contemplated where thefluoride-conducting encapsulant is conformal or has a void spaceinsufficient to fully accommodate conversion from the metal to metalfluoride, but has suitable flexibility to stretch or contract withoutrupture or cracking of the encapsulant. Such compositions may betwo-dimensional (e.g. film-void-coating), or three-dimensional (e.g.nanoparticle-void-coating, or a more complex arrangement such asmetal-impregnated zeolite-void-coating) as desired.

In still other embodiments, multiple, concentrically arrangedencapsulants are contemplated. The respective concentrically arrangedencapsulants can be separated by voids, and may be constructed of thesame or different materials. In still other embodiments ofconcentrically arranged encapsulants, the active material and theoutermost encapsulant (that contacts the electrolyte) may be separatedby a polymer or other flexible material that is able to permit thepassage of fluoride ions and is dimensionally able accommodate thevolume changes upon cycling with rupturing the outermost encapsulant.

As will be understood, an active material completely surrounded andpositioned within an encapsulant, but with at least some remaining voidspace and/or compressible non-active material (such as a polymer) can bereferred to as a “yolk-shell” nanocomposite structure. Such fullyencapsulated structures can be based on various compositionalarrangements of active material and fluoride-conducting encapsulant.However, other arrangements that include an active material onlypartially surrounded by a fluoride-conducting protective coating arealso contemplated. Such structures may include two or three dimensionalnon-fluoride conducting support structures (e.g. films, open sidedcells, tubes, or the like) containing an active material having one ormore sides coated with a fluoride conducting material to allow iontransport. Such support structures can include void space ordimensionally flexible polymer or other material to accommodate thevolume changes upon cycling without rupturing the support structure orthe encapsulant.

General preparative strategies for “yolk-shell” nanocomposite structuresare described in Lou et al., Adv. Mater., 2008, 20, 3987-4019. The metal“yolk” material discussed is commonly Au, which is considered not to bea useful active material for FIB electrochemical cells. Likewise, the“shell” material described is often SiO2, which is considered not to bea useful fluoride ion conducting material. Therefore, suitablepreparative strategies for “yolk-shell” nanocomposites useful in FIBelectrochemical cells are outlined below. These are intended to beexemplary and are not limiting of the current invention. In certainexamples, Cu metal or CuF₂ will be used as examples of the activematerial “yolk” and LaF₃ will be used as an example of an encapsulant or“shell” material; as before, these are not limiting of the invention asany material that can accommodate or release fluoride ions uponelectrochemical reaction can be envisaged to constitute the “yolk” andany phase or material that permits the exchange of F− between theelectrolyte and the active material can be envisaged to constitute theencapsulant or shell. In certain embodiments the active material is lessthan 1 micron in diameter, and most usefully, the active material “yolk”is between 1 and 500 nm in diameter and the encapsulant is from 2 to 100nm thick.

FIG. 1B outlines a general route using a sacrificial inorganic “middle”layer such as SiO₂. For example, Cu nanoparticles can be prepared byreduction of a solution of Cu²⁺ ions using hydrazine or similar reducingagent in the presence of stabilizing and/or coordinating species such assodium citrate and/or surfactant (e.g. cetyltrimethylammonium bromide).Exposure of Cu nanoparticles to a surface-modifying ligand such asaminopropyltrimethoxysilane, APTS, (or other suitable bifunctionalspecies such that one part of the molecule coordinates to the Cusurface, and the other part is presents a reactive silicon moiety to theexternal environment) followed by addition of hydrolyzable silica sourcesuch as tetraethylorthosilicate (TEOS) or sodium silicate solution(water glass) under appropriate conditions (e.g. Stöber synthesis orsol-gel reaction) results in conformal coating of the Cu nanoparticleswith SiO₂. The thickness of the SiO₂ layer (and, hence, the resultingvoid space) can be controlled by modification of the amount of SiO₂precursor used and the reaction conditions. The SiO₂-coated Cunanoparticles are then coated with an outer layer of LaF₃ by sol-gelreaction (optionally in the presence of surfactant such as Lutensol AO),whereby the thickness of this coating can be modified by the amount ofLaF₃ precursors used and the reaction conditions. This step can be doneafter separation and/or purification of the intermediate Cu@SiO₂material, or can be performed in the same reaction mixture afterformation of the SiO₂ layer. The resulting Cu@SiO₂@LaF₃ composite may,optionally, undergo an annealing step, and/or the SiO₂ layer is thenremoved by exposure of the composite to a SiO₂ etchant material such asNaOH or HF under appropriate conditions to afford the Cu@LaF₃“yolk-shell” composition. This material may subsequently undergo a finalannealing step, optionally in the presence of a reducing agent such asH₂ to purify the Cu surface.

FIG. 1C describes a similar route using a sacrificial polymer “middle”layer. For example, Cu nanoparticles are coordinated by a polymer shell,by formation of Cu nanoparticles in the presence of a polymer orcopolymer that features amino-, hydroxyl-, carboxylate or otherionizable functional groups (such as poly(acrylic acid),poly(ethyleneimine), poly(vinyl alcohol), poly(styrene sulfonate), aprotein, a polysaccharide, or gelatin), or by growth of a polymer fromthe surface of suitably-modified Cu nanoparticles (e.g. poly(styrenesulfonate) grown by atom transfer radical polymerization from a11-aminoundecyl 2-bromoisobutyrate functionalized surface). Thethickness of the polymer layer (and, hence. the resulting void space)can be controlled by the polymer concentration and/or molecular weight.A shell of LaF₃ is grown around the outside of this Cu@polymernanocomposite by sol-gel reaction to afford a Cu@polymer@LaF₃nanocomposite. The polymer layer is then removed by decomposition atelevated temperature (in air or under inert gas such as Ar) ordissolution in suitable solvent (e.g. toluene, dichloromethane oracetone) to give the desired Cu@LaF₃ “yolk-shell” composition. Thismaterial may subsequently undergo a final annealing step, optionally inthe presence of a reducing agent such as H₂ to purify the Cu surface.Alternatively, a polymer core-shell architecture such as the hollowlatex-type particles described in McDonald and Devon, Adv. Colloid.Interf. Sci., 2002, 99, 181-213 may be employed as a template in whichCu nanoparticles are entrapped (either by exposure of Cu nanoparticlesto pre-formed hollow latex particles or by coordination of Cu ions insolution to the ionizable pre-polymer or copolymer, followed byreduction of the Cu ions to give Cu nanoparticles and then generation ofthe hollow structure through e.g. solvent removal), followed by growthof LaF₃ shell, removal of polymer and annealing, as necessary, togenerate a Cu@LaF₃ “yolk-shell” composition.

FIG. 1D delineates an alternative route in the absence of a sacrificial“middle” layer. For example, Cu nanoparticles are treated with asuitable surface-modifying ligand (e.g. 11-aminoundecanoic acid, AUDA)before an outer layer of LaF₃ is grown by sol-gel reaction to give aCu@LaF₃ “core-shell” composite, which may then, optionally, subsequentlyundergo an annealing step. Partial etching of the Cu “core” using anappropriate etchant (for example, KCN, HCl/H₂O₂ or FeCl₃; suitableetchants for a wide variety of metals and compounds are given in “TheCRC Handbook of Metal Etchants” (CRC, 1990, P. Walker and W. H. Tarneds.), and may be chosen so as to not affect the “shell” material)enabled through control over the reaction conditions (e.g. etchantconcentration, temperature, reaction time) generates void space in theCu@LaF₃ particle, affording a “yolk-shell” Cu@LaF₃ composition. Thismaterial may subsequently undergo a final annealing step, optionally inthe presence of a reducing agent such as H₂ to purify the Cu surface.

FIG. 1E depicts a variation on the “sacrificial” syntheses describedabove, whereby nanoparticles of the active material are grown on thesurface of a “sacrificial” material (here, it is the innermost materialthat is removed). For example, one or more Cu nanoparticles are grown onthe surface of amino-functionalized poly(styrene) or SiO₂ particles. Theresulting composite material is treated with a suitable Cusurface-modifying ligand (e.g. AUDA), after which an outer layer of LaF₃is grown by sol-gel reaction. The innermost material is removed bythermal decomposition, etching or dissolution, resulting in a“yolk-shell” Cu@LaF₃ composition featuring one or more Cu nanoparticles.This material may subsequently undergo a final annealing step,optionally in the presence of a reducing agent such as H₂ to purify theCu surface(s) and, optionally, to aggregate the Cu nanoparticles.

FIG. 1F outlines an alternative strategy, in which active material isgrown in the internal structure or pores of a pre-formed “sacrificial”material. For example, Cu nanoparticles may be grown inside hollow SiO₂nanospheres (see e.g. Hah et al., Chem. Commun., 2004, 1012-1013 for apossible synthetic approach). These Cu@SiO₂ “core-shell” nanocompositesare then coated with an outer layer of LaF₃ by sol-gel reaction(optionally in the presence of surfactant such as Lutensol AO), wherebythe thickness of this coating can be modified by the amount of LaF₃precursors used and the reaction conditions. The resulting Cu@SiO₂@LaF₃composite may, optionally, undergo an annealing step, and/or the SiO₂layer is then removed by exposure of the composite to a SiO₂ etchantmaterial such as NaOH or HF under appropriate conditions to afford theCu@LaF₃ “yolk-shell” composition. This material may subsequently undergoa final annealing step, optionally in the presence of a reducing agentsuch as H₂ to purify the Cu surface. In related syntheses, micro- ormesoporous materials such as zeolites may be used as the “template” forCu nanoparticle formation, followed by subsequent Cu@LaF₃ “yolk-shell”generation in analogous fashion.

FIG. 1G also depicts an analogous alternative strategy whereby ahighly-electropositive metal or its metal fluoride, such as CaF₂, isgrown within the interior space of a hollow material such as apoly(styrene)-poly(acrylic acid) latex copolymer in a suitable solvent.These polymer-encapsulated CaF₂ nanocrystals are then coated with anouter layer of fluoride-ion conducting material of suitableelectrochemical stability so as not to be itself reduced at theconversion potential of CaF₂ to Ca (˜0.2 V vs. Li+/Li). Examples ofsuitable protective materials include solid solutions such asCa_(x)Ba_(y)F₂ (x+y=1) where the Ca²⁺ and Ba²⁺ ions in the protectiveshell are not substantially reduced during the conversion reaction ofCaF₂ particles within the shell; in contrast a shell featuring a lesselectropositive metallic element (e.g. LaF₃) would itself be reduced inpreference to the inner CaF₂ particles. The resultingCaF₂@polymer@Ca_(x)Ba_(y)F₂ composite may, optionally, undergo anannealing step, and/or the polymer layer is then removed by exposure ofthe composite to high temperature or a suitable solvent etchant materialthat removes the polymer to the CaF₂@Ca_(x)Ba_(y)F₂ “yolk-shell”composition.

Using the described encapsulated active material and/or yolk-shellnanocomposite electrodes, along with electrolytes, binders, additives,separators, battery casing or packaging, current collectors, electricalcontacts, electronic charge and discharge controllers, and otherelements of battery construction known to those skilled in the art, onecan create useful lithium-free electrochemical cells operable attemperatures ranging from −40 degrees to 200 degrees Celsius. Suchelectrochemical cells can have substantially irreversibleelectrochemical reactions during discharge, making them suitable forforming galvanic cells or primary batteries. Alternatively, certainstructures and compositions having an electrochemical reaction is atleast partially reversible through application of electrical charge,secondary (rechargeable) batteries can be formed.

In certain embodiments, electrolytes suitable for FIB battery systemscan include a fluoride salt and a solvent in which the fluoride salt isat least partially present in a dissolved state. The fluoride salt canbe a metal fluoride or a non-metal fluoride. The solvent can be anorganic liquid or an ionic liquid, or a mixture of the two. In otherembodiments, electrolytes suitable for FIB battery systems can include acomposite electrolyte containing fluoride salt, a polymer and optionallyan organic liquid, an ionic liquid, or a mixture of the two.Electrolytes can include, but are not limited to combinations offluoride salts and solvents disclosed in U.S. Pat. No. 9,166,249, titled“Fluoride Ion Battery Compositions”, the disclosure of which is hereinincorporated by reference.

For example, liquid electrolyte salts suitable for FIB systems maycontain complex cations in combination with the fluoride anion. Thecation may feature organic groups, such as alkylammmonium,alkylphosphonium or alkylsulfonium species, or may consist ofmetal-organic or metal-coordination complex motifs, such asmetallocenium species. Useful solvents for such liquid electrolyte saltsmay include non-aqueous solvents (denoted here as “organic”) that arecapable of dissolving the aforementioned fluoride salts to molarconcentrations of 0.01 M and above, preferred concentrations beingbetween 0.1 and 3 M. Examples of such solvents include acetone,acetonitrile, benzonitrile, 4-fluorobenzonitrile,pentafluorobenzonitrile, triethylamine (TEA), diisopropylethylamine,1,2-dimethoxyethane, ethylene carbonate, propylene carbonate (PC),γ-butyrolactone, dimethyl carbonate, diethyl carbonate (DEC), methylethyl carbonate, propyl methyl carbonate, tetrahydrofuran,2-methyltetrahydrofuran, nitromethane, benzene, toluene, chloroform,dichloromethane, 1,2-dichloroethane, dimethylsulfoxide, sulfolane,N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), carbondisulfide, ethyl acetate, methyl butyrate, n-propyl acetate, methylpropionate, methyl formate, 4-methyl-1,3,-dioxolane, pyridine, methylisobutyl ketone, methyl ethyl ketone, hexamethylphosphoramide,hexamethylphosphorus triamide, 1 methyl-2-pyrrolidinone, 2-methoxyethylacetate, trimethyl borate, triethylborate and substituted derivativesthereof, as well as sulfones such as ethylmethylsulfone, trimethylenesulfone, 1-methyltrimethylene sulfone, ethyl-sec-butyl sulfone, ethylisopropyl sulfone (EIPS), 3,3,3-trifluoropropylmethyl sulfone,2,2,2-trifluoroethyl sulfone, bis(2,2,2-trifluoroethyl)ether (BTFE),glymes (e.g., diglyme, tetraglyme), 1,2-dimethoxyethane (DME) andmixtures thereof. In certain embodiments, room temperature ionic liquidmaterials, or ionic liquids that remain liquid at temperatures below 200degrees Celsius (such as those described in “Electrochemical Aspects ofIonic Liquids”, E. Ohno ed., Wiley Interscience, New York, 2005), arepreferred. These can include ionic liquids that remain liquid attemperatures below 100 degrees Celsius such as 1-methyl,1-propylpiperidinium bis(trifluoromethanesulfonyl)imide (MPPTFSI),butyltrimethylammonium bis(trifluoromethanesulfonyl)imide (BTMATFSI) and1-butyl, 1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide(BMPTFSI) and their fluoroalkylphosphate (FAP) anion derivatives (e.g.MPPFAP) where FAP is a hydrophobic anion such astris(pentafluroethyl)trifluorophosphate, all of which alone or incombination are useful solvents.

In certain other embodiments, the electrolytes suitable for FIB batterysystems can include the compositions disclosed above with the additionof a fluoride-ion complexing species such as an anion receptor, a cationcomplexing species such as a crown ether, or a combination of both.Suitable anion receptors include species capable of binding fluorideanion such as boron, aluminum, ammonium, H-bond donor or similar groups,including aza ethers and alkyl and aryl boron and boronate complexessuch as those described in McBreen et al, J. Power Sources, 2000, 89,163 and West et al., J. Electrochem. Soc., 154, A929 (2007), and boroxinspecies such as those described in Nair et al., J. Phys. Chem. A, 113,5918 (2009), all of which are incorporated by reference herein. Inparticular tris(hexafluoroisopropyl)borate,tris(pentafluorophenyl)borane and all possible regioisomers ofdifluorophenyl boroxin (DFB), trifluorophenyl boroxin,bis(trifluoromethyl)phenyl boroxin, trifluoromethyl)phenyl boroxin andfluoro(trifluoromethyl)phenyl boroxin can be used.

As will be appreciated, fluoride ion batteries are suitable for a widerange of primary or rechargeable applications, including but not limitedto vehicle traction batteries (electric vehicles (EV), hybrid vehicles(HEV), and plug-in hybrid (PHEV)) or vehicle starter or ignitionbatteries. FIB systems can be useful stationary batteries for emergencypower, local energy storage, starter or ignition, remote relay stations,communication base stations, uninterruptible power supplies (UPS),spinning reserve, peak shaving, or load leveling, or other electric gridelectric storage or optimization applications. Small format or miniaturebattery applications including watch batteries, implanted medical devicebatteries, or sensing and monitoring system batteries (including gas orelectric metering) are contemplated, as are other portable applicationssuch as flashlights, toys, power tools, portable radio and television,mobile phones, camcorders, lap-top, tablet or hand-held computers,portable instruments, cordless devices, wireless peripherals, oremergency beacons. Military or extreme environment applications,including use in satellites, munitions, robots, unmanned aerialvehicles, or for military emergency power or communications are alsopossible.

III (c) Comparative Example 1 and Experimental Example 1

In a Comparative Example 1, copper nanoparticles were made without ashell and analyzed. First, 2 mmol of Cu(NO₃)₂.2.5H₂O and 1.87 mmol CTABwere dissolved in 75 ml water at room temperature, and 0.5 ml NH₄OH(28-30 wt % NH₃ in water, 14.8M) were added to adjust the pH to about10-11. A solution was prepared containing hydrazine (3 ml of 50-60%reagent grade), CTAB (1.87 mmol), and citric acid (0.38 mmol) in water(75 ml) under argon and mixed for about 20 minutes before adding thecopper nitrate solution. The reaction mixture was stirred for 1.5 hrs,to maximize copper nanoparticle growth. The resulting coppernanoparticles (˜50 nm) were isolated and washed. Specifically, thereaction synthesis mixture was centrifuged, decanted, mixed with ethanoland sonicated. FIG. 3 shows an X-ray diffraction (XRD) spectrum of thecopper nanoparticles as-made. Three peaks are visible, all correspondingto Cu (° 2θ): 43.0, 50.5, and 74.0. However, upon exposure to air, Cu isoxidized to Cu₂O, which begins forming at least as early as 4 days andis the main product after 9 days. This is illustrated in FIG. 4, whichshows appearance of new peaks at 29.5, 42.3, 61.3, and 73.5°2θ,corresponding to Cu₂O.

In Experimental Example 1, core-shell nanoparticles were made inaccordance with the present disclosure that comprised a core comprisinga copper nanoparticle coated with a shell comprising lanthanum fluoride(Cu/LaF₃). The ˜50 nm copper nanoparticles were made using the samemethod as Comparative Example 1, but were redispersed in water with 3 mlof hydrazine (3 ml of 50-60% reagent grade) under an argon atmospherefollowing isolation and washing of the copper nanoparticles. To themixture of water, copper nanoparticles, and hydrazine was added asolution of La(NO₃)₃.6H₂O (1 mmol in 15 ml H₂O) and a solution of NaF (1mmol in 15 ml H₂O). The reaction mixture was stirred for 10 minutes andthen centrifuged.

The precipitate was isolated by centrifuge and analyzed by XRD. The XRDspectrum of the core-shell nanoparticles as synthesized is shown in FIG.5. The XRD spectrum shows 5 peaks (° 2θ): 25.0 (LaF₃), 28.0 (LaF₃), 43.5(Cu), 50.4 (Cu), 74.0 (Cu). FIG. 6 shows stacked XRD spectra of thecore-shell nanoparticles upon exposure to air for 9, 16, and 23 days. Incontrast to Comparative Example 1, no spectral changes were observed.FIGS. 7A and 7B show TEM images of the core-shell nanoparticles assynthesized. As shown, the copper nanoparticle cores are covered withthe LaF₃ shell. The shell has a thickness of about 0.30 nanometers.FIGS. 8A-8C show high-resolution TEM images of the core-shellnanoparticles as synthesized. The central black areas correspond to thecopper core, and the peripheral black and white areas correspond to theLaF₃ shell. The figures show homogeneous coverage of the copper coredirectly coated with the LaF₃ shell.

Accordingly, the core-shell nanoparticle synthesized in ExperimentalExample 1 provides a shell capable of protecting the underlying metalcore. Such a core-shell nanoparticle is useful for applications wherethe operating conditions would dissolve, oxidize, or otherwisecontaminate the metal core. Illustrative examples include use of thecore-shell nanoparticles as battery electrode materials.

In a non-limiting example as shown in FIG. 9, the core-shellnanoparticle of Experimental Example 1 may be included as an activematerial in a negative electrode (anode) of an F-shuttle battery. TheLaF₃ shell protects the copper core allowing it to operate as an activematerial without being dissolved. As shown in FIGS. 10A and 10B, thecore-shell nanoparticles of Experimental Example 1 were tested as theactive material in an anode. The anode included the core-shellnanoparticles, a conductive agent (super P carbon), and PVdF binder in aratio of 8:1:1.

Comparative Example 2

An attempt was made to make a core-shell nanoparticle including a corecomprising a copper nanoparticle directly coated with a shell comprisinglanthanum fluoride (Cu/LaF₃). Comparative Example 2 was performedidentically to Experimental Example 1, except that 1 mmol LaCl₃.7H₂O wasused instead of La(NO₃)₃.6H₂O.

The XRD spectrum of the nanoparticles as synthesized in ComparativeExample 2 is shown in FIG. 11. The XRD spectrum shows 5 peaks (° 2θ):24.5 (LaF₃), 27.6 (LaF₃), 43.6 (Cu), 50.5 (Cu), 74.1 (Cu). Therefore,the Cu remains during the course of the reaction of LaCl₃ and NaF.

However, oxidation of the Cu following exposure of the core-shellnanoparticles of Comparative Example 2 indicated that the shell was notproperly formed. FIG. 12 shows stacked XRD spectra of the nanoparticlessynthesized in Comparative Example 2 upon exposure to air for 8, 15, and22 days. Additional peaks are observed starting at 8 days (° 2θ): 35.4,36.4, 38.8, 42.5, 44.8, 48.7, 52.3, 61.5, 73.5. At least the peaks at36.4, 42.5, 61.5, and 73.5°2θ are consistent with Cu₂O formation. Thepeaks at 43.6, 50.5, and 74.1°2θ, which are consistent with Cu, havealso diminished in intensity. As shown in FIG. 13, the TEM image showsinhomogeneous, partial coverage of Cu nanoparticles with LaF₃, as wellas LaF₃ that is not associated with Cu nanoparticles. Accordingly,shells made with LaCl₃.7H₂O do not result in a desirable core-shellcomposition as it would leave the Cu core exposed to the environment ofan electrochemical cell that might dissolve the Cu core. In addition,the LaF₃ that is not associated with Cu nanoparticles would decrease theoverall efficiency of any system incorporating the mixture.

Layered Thin Film Electrode Study

The core and shell materials may also be studied in a thin filmelectrode configuration. Briefly, copper was sputtered to a thickness ofabout 80 nm onto a glassy carbon substrate of 1 mm thickness. Afterformation of a Cu film, LaF₃ was sputtered onto the Cu layer to athickness of less than 5 nm, to form a double-layered thin film; forcomparison purposes, a single layered Cu thin film without the LaF₃coating was also prepared. The thin films were studied in a threeelectrode cell configuration, with a Ag-wire soaked in1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide(MPPyTFSI) and 0.01 M AgOTf as the reference electrode and a Pt-wire asa counter electrode, and using 0.1M tetramethylammonium fluoride (TMAF)in MPPyTFSI as the electrolyte. Cyclic voltammetry was measured in therange from −2.4V to −0.7V vs. Ag/Ag+. Evaluation was conducted in amoisture- and oxygen-free glove box.

The cyclic voltammograms obtained are shown in FIG. 14. For the doublelayered thin film, the anodic peak measured was symmetrical with thecathodic peak, and no copper ions were detected in the electrolyte byICP-MS. These results indicate reversible transport of fluoride ionsfrom the LaF₃ layer into the Cu later. In comparison, the cyclicvoltammogram obtained for the single layered Cu thin film is asymmetric.The anodic current exceeds the cathodic current, indicating dissolutionof the Cu during anodic reaction. ICP-MS data confirms this, showing 5ppm Cu in the electrolyte.

The Cu—LaF₃ double layered thin film electrode was also studied by XPS,initially and after fluorination at the voltages indicated in FIG. 15A.The initial time point (1 in FIG. 15A) was studied as deposited. Thepotential was then swept from OCV (about −1.5V vs Ag/Ag+) to −0.8V andmaintained for 1 hour. After fluorination reaction, a sample of theelectrode was taken for XPS depth profiling analysis.

In the initial XPS spectrum (FIG. 15B) the surface contains more La andF than Cu. After fluorination, fluoride ions can be detected at higherlevels at deeper depths than in the initial XPS spectrum; in the initialspectrum, fluoride levels dropped off more sharply with increasingdepth. Collectively, the XPS data show that fluoride ions can penetrateinto the copper layer. Thus, after reduction, fluoride ions can diffuseinto the Cu cores.

While the aspects described herein have been described in conjunctionwith the example aspects outlined above, various alternatives,modifications, variations, improvements, and/or substantial equivalents,whether known or that are or may be presently unforeseen, may becomeapparent to those having at least ordinary skill in the art.Accordingly, the example aspects, as set forth above, are intended to beillustrative, not limiting. Various changes may be made withoutdeparting from the spirit and scope of the disclosure. Therefore, thedisclosure is intended to embrace all known or later-developedalternatives, modifications, variations, improvements, and/orsubstantial equivalents.

Thus, the claims are not intended to be limited to the aspects shownherein, but are to be accorded the full scope consistent with thelanguage of the claims, wherein reference to an element in the singularis not intended to mean “one and only one” unless specifically sostated, but rather “one or more.” All structural and functionalequivalents to the elements of the various aspects described throughoutthis disclosure that are known or later come to be known to those ofordinary skill in the art are expressly incorporated herein by referenceand are intended to be encompassed by the claims. Moreover, nothingdisclosed herein is intended to be dedicated to the public regardless ofwhether such disclosure is explicitly recited in the claims. No claimelement is to be construed as a means plus function unless the elementis expressly recited using the phrase “means for.”

Further, the word “example” is used herein to mean “serving as anexample, instance, or illustration.” Any aspect described herein as“example” is not necessarily to be construed as preferred oradvantageous over other aspects. Unless specifically stated otherwise,the term “some” refers to one or more. Combinations such as “at leastone of A, B, or C,” “at least one of A, B, and C,” and “A, B, C, or anycombination thereof” include any combination of A, B, and/or C, and mayinclude multiples of A, multiples of B, or multiples of C. Specifically,combinations such as “at least one of A, B, or C,” “at least one of A,B, and C,” and “A, B, C, or any combination thereof” may be A only, Bonly, C only, A and B, A and C, B and C, or A and B and C, where anysuch combinations may contain one or more member or members of A, B, orC. Nothing disclosed herein is intended to be dedicated to the publicregardless of whether such disclosure is explicitly recited in theclaims.

The examples are put forth so as to provide those of ordinary skill inthe art with a complete disclosure and description of how to make anduse the present invention, and are not intended to limit the scope ofwhat the inventors regard as their invention nor are they intended torepresent that the experiments below are all or the only experimentsperformed. Efforts have been made to ensure accuracy with respect tonumbers used (e.g. amounts, dimensions, etc.) but some experimentalerrors and deviations should be accounted for.

Moreover, all references throughout this application, for example patentdocuments including issued or granted patents or equivalents; patentapplication publications; and non-patent literature documents or othersource material; are hereby incorporated by reference herein in theirentireties, as though individually incorporated by reference.

What is claimed is:
 1. An electrochemically active structure comprising:a core having an exterior surface, wherein the core comprises an activematerial comprising one or more metal-containing nanoparticles; and ashell at least partially surrounding the exterior surface of the core,wherein the shell consists of a metal fluoride, and wherein the shellforms an exterior surface of the electrochemically active structure. 2.The electrochemically active structure of claim 1, wherein the metal ofthe one or more metal-containing nanoparticles comprises an alkalineearth metal.
 3. The electrochemically active structure of claim 1,wherein the metal of the one or more metal-containing nanoparticlescomprises a transition metal.
 4. The electrochemically active structureof claim 1, wherein the shell is directly attached to the exteriorsurface of the core.
 5. The electrochemically active structure of claim1, wherein the active material comprises multiple particles, and theshell surrounds the multiple particles.
 6. The electrochemically activestructure of claim 1, wherein the metal of the one or moremetal-containing nanoparticles is selected from cobalt, bismuth, copper,lead, calcium, magnesium, lanthanum, and combinations thereof; andwherein the one or more metal-containing nanoparticles optionallycomprise a fluoride of the metal.
 7. The electrochemically activestructure of claim 1, further comprising a polymer encapsulant betweenthe shell and the active material.
 8. The electrochemically activestructure of claim 1, wherein the one or more metal-containingnanoparticles comprises at least one of iron, cobalt, nickel, copper,and lead.
 9. The electrochemically active structure of claim 8, whereinthe one or more metal-containing nanoparticles comprises at least one ofcobalt and copper.
 10. An electrode comprising the electrochemicallyactive structure of claim 1, wherein: the metal of the one or moremetal-containing nanoparticles comprises a first metal; and wherein themetal of the metal fluoride is a second metal.
 11. The electrode ofclaim 10, wherein the first metal and the second metal are differentmetals.
 12. The electrode of claim 10, wherein the first metal comprisesCu and the metal fluoride comprises LaF₃.
 13. A battery comprising theelectrode of claim
 10. 14. A battery comprising the electrode of claim12.
 15. A battery, comprising: an electrode, comprising theelectrochemically active structure of claim 1.